This dishwasher was installed with the drain hose going straight down through the floor. It still has the drain loop on the left side of the d/w, but water is still siphoning out. Can I just put some type of air gap in the drain hose or should I tell the customer to call a plumber and attach the hose to the drain right under the sink? The house has a basement, so accessing the drain hose is not a problem. It ties into the sink drain, but it's below the floor instead of right under the sink like most.

yes,by all means,attach the drain hose to the sink drain and above the p-trap,otherwise,you have a constant vacuum which will never allow the machine to contain water for any lenght of time-it is also recommended that you raise the drain hose to at least the height of the d/w tub before draining- the loop on the side of the tub is there for operation of the d/w and has nothing to do with drainage

Tarheel, Iemphatically refrain from any installation myself,but if your going to do this one thing I'd recommend is wrapping a piece of wire around the hose where it loops thru the cabinet to keep the hose from kinking in a few years .